‘Eve’ Gets Support as Tie That Binds Us All : New Genetic Evidence Bolsters Theory That Everyone Descends From 1 African Woman

New genetic evidence has strengthened the claim that all humans alive today are descended from a single African woman and suggests that the woman lived 140,000 years ago, not 200,000 as was previously believed, a UC Berkeley researcher said in San Diego Tuesday evening.

Descendants of that woman, whom scientist have dubbed “Eve,” started migrating north out of Africa 75,000 years ago and had fully settled in southern Europe 35,000 years ago, displacing the indigenous Neanderthal population, said molecular biologist Allan C. Wilson. The Neanderthals were dispossessed because they were genetically incompatible with the humans of African origin, Wilson said at an international genetics conference.

The new evidence also indicates that early males may have roamed rather freely throughout Africa nearly 100,000 years ago while females stayed firmly rooted, suggesting that sharp changes in cultural interaction of men and women occurred over a period of about 30,000 years.

Wilson reached his conclusions by studying deoxyribonucleic acid (DNA, the genetic blueprint of humans) found not in the nucleus of cells, where most genes are stored, but in mitochondria, small bodies inside the cell that are the source of the cell’s energy. Mitochondria have only 37 genes, compared to the estimated 100,000 found in the cell’s nucleus, making them much easier to study.

Unlike genes in the nucleus, which are inherited from both parents, genes in mitochondria are passed down only from mothers. Because of this, researchers can trace maternal lineages unaffected by contributions from males.

Mutations in mitochondrial DNA are believed to occur at a constant rate over long periods of time. By studying the differences between mitochondrial DNA from two women, for example, researchers can determine how long ago they shared a common female ancestor. When many women are used, the technique also reveals where that ancestor lived.

Using mitochondrial DNA from placentas, Wilson and his colleagues reported in 1987 that all modern humans are descended from a woman who lived in southern Africa about 200,000 years ago, a woman who has subsequently become known as Eve or “the mother of us all.” That finding does not mean that only one woman was alive then. Rather, it means that the lineages that descended from all other women eventually died out--a phenomenon that is common in biology.

By comparing every possible pairing among the 147 people tested in the study, the researchers concluded that Eve could only have lived in Africa.

Although other researchers generally accept Wilson’s methodology, they criticized two aspects of his studies: the fact that he used black Americans in his study as a surrogate for Africans, and the speed of his so-called “molecular clock.” The molecular clock is a measure of the rate at which mutations occur in mitochondrial DNA, and critics charged that Wilson’s was too fast, placing Eve too near us in time.

In his current study, Wilson used newly developed biochemical techniques that allowed him to study the mitochondrial DNA contained in a sample as small as a single hair. Because he could use hairs, he was able to obtain specimens from two hunter-gatherer tribes in central Africa and from the !Kung bushmen of southern Africa, who would not have donated placentas for such a study. (The spelling of !Kung denotes a clicking sound in the pronunciation of the K.)

He was also able to perform similar studies on chimpanzees, which helped him achieve a finer calibration of his molecular clock.

“We have been able to refute both criticisms,” Wilson told a meeting of geneticists.

Based on the new data, he said, Eve lived in southern Africa 142,000 years ago. “We are all a derivative of one of the !Kung lineages,” he said.

The data further show that humans--men and women together--began migrating out of Africa into the Middle East about 75,000 years ago. By 35,000 years ago, they were firmly established in southern Europe, displacing the Neanderthals who had lived there.

That agrees with archeological data, thereby confirming the accuracy of his molecular clock. The fact that Neanderthal women made no contribution to the gene pool indicates, he said, that a fundamental genetic incompatibility existed between the two groups.

Although men and women began migrating out of Africa 75,000 years ago, men must have begun moving around and exploring the continent much earlier, he said. Evidence for this was provided by the genes of the three groups of African women.

Men Did the Exploring

In those stages men did all the exploring and left women to tend to the home, whereas later the family group was clearly more important and migrated as a unit.

Wilson found virtually no overlap in the mitochondrial DNA from the three groups, indicating that the females had stayed put for periods long enough for gene differences to develop--many tens of thousands of years. To his surprise, however, Wilson found an almost complete overlap in key nuclear genes, which are inherited from both parents.

“This means that during most of the time in Africa, our female ancestors moved very little, while the males apparently migrated quite a lot,” he said.